This invention lies in the field of durable wood bonding adhesives comprising phenol-resorcinol-formaldehyde polymers. It is specifically concerned with improved hardener formulations for these resins.
Room temperature setting phenol-resorcinol-formaldehyde (PFR) resins have been in use for durable wood bonding for approximately four decades. These resins are commonly sold as water-based liquid compositions. They are commonly referred to as novolacs since they require an additional source of methylene groups to effect curing to a cross-linked insoluble product. A methylene group donating curing agent is commonly added to the resin immediately before use to create a wood adhesive. Depending on the composition of the resin and hardener, the mixed adhesive can have a pot life varying between 30 minutes and several hours. The methylene group donor will typically be formaldehyde or one of its water soluble polymers known generally as paraformaldehyde or alpha-polyoxymethylene. The hardener will also normally contain inert materials which serve to control the viscosity and penetration of the mixed adhesive.
Until recent years, most hardeners for PFR resins have been powdered solid materials. These powdered hardeners were mixed batch wise into the resin at the point of use. Typical resins and hardener formulations will be similar to those described in U.S. Pat. No. 3,389,125 to Dietrick et al.
There are numerous disadvantages in using PFR adhesives in batch wise fashion. Because a finite time is required to use a given batch of adhesive, the freshly mixed adhesive will have different characteristics from the adhesive near the end of its useful pot life. This is due to gradual advancement of the adhesive toward its final cured state. Pot life of the adhesive is simply the permissible period of time before is becomes too highly advanced for use.
In an effort to overcome the problems of advancement of the adhesive, liquid hardeners were developed which could be continuously mixed with the resin. Examples of such resin and hardener systems are found in U.S. Pat. Nos. 3,634,276 to Kreibich et al and 4,061,620 to Gillern. When liquid resins are continuously mixed at the point of use with liquid hardeners there is essentially no inventory of mixed resin which can cause pot life problems and variation in adhesive characteristics. Simple and effective mixers have been developed so that the adhesive is mixed on demand. An example of such as mixture is found in U.S. Pat. No. 3,938,467 to Radowicz. Similar mixers to continuously blend a powdered hardener into a liquid resin have also been developed, as evidenced by U.S. Pat. No. 4,201,485 to Walker. However, the so-called continuous liquid-powder blenders have experienced many technical problems and have not found wide acceptance in industry.
Systems using liquid resins and liquid hardeners (liquid-liquid systems) are not without their own problems. A principle trouble has been variation in the characteristics of the hardener over time. The original liquid hardeners contained formalin, but were soon abandoned because of the objectionable formaldehyde odor. This problem was somewhat solved by using paraformaldehyde and later by using alpha-polyoxymethylene to reduce odor. These materials tend to depolymerize over time, yielding monomeric formaldehyde. As a result, the odor increases to objectionable levels and the curing characteristics of the adhesives also change. Pot life and cure time will be shorter when a significant amount of monomeric formaldehyde is present. This effect begins to be noticed immediately after the liquid hardeners are mixed. Within one or two days the formaldehyde odor problem can be severe. Thus, to date there have been unsolved problems for the wood laminator whether he chooses to use a batch liquid-powder system, a continuously mixed liquid-powder system, or a continuously mixed liquid-liquid system. The continuously mixed liquid-liquid system appears to be in the acendency, but the problems of stability and odor have yet to be fully overcome.